1. Technical Field
The present invention relates to a self-excited switching power supply circuit, and more specifically, to a flyback self-excited switching power supply circuit in which energy stored in a transformer is emitted from a secondary output winding to a load when an exciting current flowing in a primary winding of the transformer is stopped.
2. Description of the Related Art
A switching power supply circuit functioning as a stabilizing circuit for converting a commercial AC voltage to a DC voltage and outputting the resultant voltage is used in a battery charger, a motor and the like provided with a DC-DC converter. A driving system (switching system) of a switching element is roughly divided into a self-excited oscillation system and a separately-excited oscillation system. The self-excited oscillation system performs oscillating operation by feeding a voltage appearing across a feedback winding of an inductance member such as a transformer back positively as a driving signal to a control terminal of a switching element. A self-excited switching power supply circuit employing the self-excited oscillation system is known conventionally (Japanese Patent No. 36914798).
A conventional self-excited switching power supply circuit 100 is described with reference to FIG. 8. In the self-excited switching power supply circuit 100, a capacitor-input rectifying and smoothing circuit with a bridge rectifier 7 and a smoothing capacitor 1 is connected to a commercial AC power supply AC through a power supply switch 4. Further, an unstable DC voltage obtained by rectifying and smoothing a commercial AC voltage is generated between a high-voltage side terminal 1a and a low-voltage side terminal 1b of the smoothing capacitor 1, and the generated voltage is used as a DC input power supply.
Reference numeral 2 shows a transformer including a primary winding 2a, a first feedback winding 2b wound in the same direction as the primary winding 2a, and a second feedback winding 2d wound in the opposite direction to the primary winding 2a that are provided on the primary side, and a secondary output winding 2c provided on the secondary side. Reference numeral 3 shows an oscillation field effect transistor (hereinafter abbreviated as FET). Reference numeral 21 shows a start-up resistor used to apply a forward bias (in other words, a gate voltage being equal to or higher than a threshold voltage VTH) to the gate of the FET 3 when the circuit is started. An electric resistor 25 connected in series to the start-up resistor 21 has a resistance value smaller than that of the start-up resistor 21. Thus, when the charging voltage of the smoothing capacitor 1 (input voltage of DC input power supply) is divided and if a low DC voltage is output, the circuit is not started.
Reference numeral 6 shows a Zener diode provided to prevent excessive input to the gate of the FET 3. Reference numeral 12 shows a start-up capacitor connected in series to a feedback resistor 11 and connected between the feedback winding 2b and the gate of the FET 3. Reference numeral 24 shows an electric resistor provided to prevent excessive input to the gate of the FET 3, and reference numeral 5 shows an OFF control transistor element having a collector connected to the gate of the FET 3, and an emitter connected to the low-voltage side terminal 1b. The base of the OFF control transistor 5 is connected to a junction between the FET 3 and a primary current detecting resistor 51 through an electric resistor 52, and to the low-voltage side terminal 1b through an OFF control capacitor 53.
An output side rectifying and smoothing circuit 26 with a rectifying diode and a smoothing capacitor is connected to the secondary output winding 2c. The output side rectifying and smoothing circuit 26 rectifies and smoothes the output of the secondary output winding 2c, and gives the resultant output between a high-voltage side output line 20a and a low-voltage side output line 20b. 
In the self-excited switching power supply circuit 100 of the aforementioned structure, when the power supply switch 4 is turned on to connect the self-excited switching power supply circuit 100 to the commercial AC power supply AC and start the self-excited switching power supply circuit 100, the smoothing capacitor 1 is charged with a sinusoidal voltage obtained by full-wave rectification at the bridge rectifying circuit 6. Then, an input voltage of the DC input power supply that increases from 0 V to 141 V appears as shown in FIG. 9 between the high-voltage side terminal 1a and the low-voltage side terminal 1b of the smoothing capacitor 1. In response to the increase of the input voltage of the DC input power supply, the start-up capacitor 12 (in FIG. 8, the lower electrode is a positive electrode and the upper electrode is a negative electrode) is charged through the start-up resistor 21. This gradually increases the charging voltage of the start-up capacitor 12 to increase the gate voltage of the FET 3 as shown in FIG. 11. When the charging voltage of the start-up capacitor 12 reaches the threshold voltage VTH and a forward bias voltage is applied to the gate of the FET 3, the FET 3 is turned on.
When the FET 3 is turned on and an exciting current starts to flow from the smoothing capacitor (DC input power supply) 1 to the primary winding 2a connected in series to the FET 3, induced electromotive force is generated in each winding of the transformer 2 to store exciting energy in the transformer 2. In response to increase of a current flowing in the primary winding 2a, the OFF control capacitor 53 is charged with a voltage across the primary current detecting resistor 51 to increase the base voltage of the OFF control transistor 5. An induced voltage generated in the feedback winding 2b is superimposed on the charging voltage of the start-up capacitor 12 during the ON period of the FET 3, so that the gate voltage of the FET 3 is maintained at the threshold voltage VTH of the FET 3 or a level higher than the threshold voltage VTH.
A current flowing in the primary winding 2a increases in proportion to time elapsed after the FET 3 is turned on. The charging voltage of the OFF control capacitor 53 to be charged with a voltage across the primary current detecting resistor 51 also increases. If the base voltage of the OFF control transistor 5 reaches a predetermined bias voltage, the OFF control transistor 5 performs ON operation. This provides continuity between the collector and the emitter of the OFF control transistor 5 to bring the gate of the FET 3 into a state where the gate of the FET 3 is substantially shorted with the low-voltage side terminal 1b, thereby turning the FET 3 off.
When the FET 3 is turned off and a current flowing in the transformer 2 is interrupted substantially, a voltage what is called a flyback voltage (induced counter-electromotive force) is generated in each winding. The flyback voltage generated in the secondary output winding 2c is rectified and smoothed by the output side rectifying and smoothing circuit 26, and is output as electric power to be supplied to a load connected between the output lines 20a and 20b. During the OFF period of the FET 3, the start-up capacitor 12 is charged with a flyback voltage generated in the feedback winding 2b and supplied through a charging path formed by the Zener diode 6 and the feedback resistor 11, and with the charging voltage (input voltage of the DC input power supply) of the smoothing capacitor 1 supplied through a charging path formed by the start-up resistor 21.
If emission of electric energy stored in the secondary output winding 2c is finished by the induced counter-electromotive force, the flyback voltage of the feedback winding 2b functioning as a reverse bias for the gate of the FET 3 drops. Then, the charging voltage supplied to the start-up capacitor 12 during the OFF period of the FET 3 makes the gate voltage of the FET 3 exceed the threshold voltage VTH to turn the FET 3 on again. A series of the aforementioned oscillating operation is repeated.
As described above, the start-up capacitor 12 is charged with the flyback voltage generated in the feedback winding 2b and through the smoothing capacitor 1 during the OFF period of the FET 3. However, the flyback voltage substantially proportionate to the input voltage of the DC input power supply is low at a moment immediately after the self-excited switching power supply circuit 100 is connected to the commercial AC power supply AC and started. Accordingly, the start-up capacitor 12 is charged mainly with the DC input power supply appearing across the smoothing capacitor 1 through the start-up resistor 21.
Meanwhile, a discharging current always flows from the smoothing capacitor 1 to the start-up resistor 21 irrespective of the operating condition of the FET 3. Accordingly, a resistor having a high resistance value of 10 MΩ or higher is used to reduce power consumption by the start-up resistor 21. Thus, the start-up capacitor 12 is charged at a low speed when the self-excited switching power supply circuit 100 is started, and intermittent oscillation is repeated that always involves an OFF period of from about 0.7 msec to about 1.8 msec as shown in FIGS. 10 and 11.
The self-excited switching power supply circuit 100 includes a current monitoring circuit 54 for monitoring an output current flowing in the output lines 20a and 20b, and a voltage monitoring circuit 55 for monitoring an output voltage between the output lines 20a and 20b. The self-excited switching power supply circuit 100 further includes an output control circuit 56 for controlling the output current and the output voltage at levels not exceeding a constant set current and a constant set voltage, respectively, at the primary side of the transformer 2 on the basis of values obtained as a result of the monitoring.
To be specific, the output control circuit 56 includes a driving capacitor 37 to be charged with the flyback voltage generated in the second feedback winding 2d of the transformer 2 during the OFF period of the FET 3. If the output current flowing in the high-voltage side output line 20a and the low-voltage side output line 20b, or the output voltage between the output lines 20a and 20b exceeds the set current or the set voltage, a photocoupler light emitting element 35 emits light. A photocoupler light receiving element 36 is optically coupled to the photocoupler light emitting element 35, and causes a discharging current proportionate to the amount of light received by the photocoupler light receiving element 36 to flow from the driving capacitor 37 to the base of the OFF control transistor 5.
Accordingly, after the FET 3 is turned on, a voltage generated in the primary current detecting resistor 51 as a result of flow of a current in the primary winding 2a, and a voltage generated in the electric resistor 52 as a result of flow of the aforementioned discharging current, are applied together to the base of the OFF control transistor 5, thereby increasing the base voltage at a higher speed. Accordingly, the OFF control transistor 5 performs ON operation promptly to turn the FET 3 off, thereby shortening turn-on time. Further, the output current or the output voltage is reduced to the set current or the set voltage, or to a level lower than the set current or the set voltage, thereby achieving constant current control and constant voltage control.
The set voltage is set at the operating voltage of the load connected between the output lines 20a and 20b. In order to protect each circuit element of the self-excited switching power supply circuit 100, the set current is set at a value higher than the operating current that allows the load to operate stably.
When the power supply switch 4 is turned on to connect the self-excited switching power supply circuit 100 to the commercial AC power supply AC and start the self-excited switching power supply circuit 100, the output current and the output voltage of the self-excited switching power supply circuit 100 repeats the aforementioned intermittent oscillating operation to increase the output current gradually. If the output current reaches the set current, the output voltage increases gradually under the constant current control as indicated by PS′ of FIG. 12. If the output voltage reaches the set voltage thereafter corresponding to the operating voltage of the load, the output current decreases gradually under the constant voltage control. If the output current reaches the operating current of the load, the output voltage and the output current appropriate for the load are output stably while the oscillating operation is repeated.
Various loads may be connected between the output lines 20a and 20b of the conventional self-excited switching power supply circuit 100. However, regarding a load such as a motor and a DC-DC converter, an operating voltage and an operating current thereof are not proportionate to each other until the load is put into stable operation. As seen from L of FIG. 12, for example, the operating current of such a load at initial time is high at a level of about 0.4 A, and is reduced slowly thereafter in response to increase of the operating voltage. The load operates stably at the operating voltage of 5.5 V and the operating current of about 0.18 A (S1 of FIG. 12).
Meanwhile, the self-excited switching power supply circuit 100 repeats the intermittent oscillating operation immediately after the self-excited switching power supply circuit 100 is connected to the commercial AC power supply AC and started. As seen from PS′ of FIG. 12, the output characteristics of the self-excited switching power supply circuit 100 are such that an initial output current (short-circuit current) after the self-excited switching power supply circuit 100 is started is low at a level of about 0.02 A, and that an output current increases while an output voltage of about 0.4 V is maintained. Accordingly, if an output current increases while intermittent oscillating operation is repeated as shown by S2 of FIG. 12, a corresponding output voltage and the output current of S2 agree with the operating characteristics of a load indicated by L of FIG. 12. In this case, electric power exceeding the output indicated as S2 is not required, so that the intermittent oscillating operation is repeated. As a result, the output of the self-excited switching power supply circuit 100 will not reach S1 of FIG. 12 at which the load operates stably, causing what is called a start-up failure.
This start-up failure may be prevented by increasing a speed at which the start-up capacitor 12 is charged immediately after the self-excited switching power supply circuit 100 is started to shorten an OFF period of the intermittent oscillating operation, thereby increasing an initial output current (short-circuit current). However, this in turn requires reduction of the resistance value of the start-up resistor 21. Using the start-up resistor 21 of a reduced resistance value generates flow of a current even in a standby state where a load is not connected, leading to a different problem of increase of standby power consumption.